The anastomosis of very small blood vessels (< 2mm diameter) has expanded the ability of surgeons to reconstruct missing tissues and to revascularize or transplant organs. However, microsurgical manipulation imposes a unique vascular trauma which enhances the risk of local thrombosis and tissue loss. Thrombi in this setting are platelet-rich, tend to occur at or within 1-2cm of the anastomosis, and are remarkably resistant to conventional antithrombotic therapy. Fortunately, the risk interval for such thromboses is relatively short (maximal in the first thirty minutes). Recently developed antibodies which when given systematically bind to the receptors on circulating platelets and prevent adhesion may be powerful enough to prevent platelet deposition at microanastomoses. Unfortunately, they prolong the bleeding time and may thereby increase the risk of postoperative bleeding. In contrast, LOCAL therapy designed to disrupt platelet adhesion to a microanastomosis is theoretically attractive by virtue of its specificity and low risk of systemic bleeding complications. It is particularly attractive in microsurgery because the anastomosis is the prime site for thrombosis and because the site at risk is open and available for incubation with local reagents during completion of the anastomosis. A microanastomosis bears three potential surfaces which may bind platelets: the injured/stimulated endothelium; exposed subendothelium at sites of injury; and suture material. Platelet adhesion may also be augmented by surface-bound thrombin at these sites. Using a fresh human vessel model, we plan to 1.) measure the relative amounts of proteins known to bind platelets on each of the three surfaces (endothelium, subendothelium and nylon suture) after exposure of each to standard, clinically relevant microsurgical manipulations; 2.) measure platelet deposition to each of these surfaces in the presence and absence of antibodies which mask their respective ligands for platelets; and 3.) based on the results of 2.), the appropriate set of inhibitory reagents will be used to prevent platelet deposition to a completed microanastomosis, which incorporates all of the respective surfaces in a clinically relevant model. Bound thrombin activity on each surface will be measured after plasma exposure, to determine the supplemental need for local inhibition of thrombin at microanastomoses. Based on these data, we hope to outline a specific collection of masking molecules which may be applied in minute amounts to open vessels at the time of anastomosis, thereby reducing the risk of microanastomotic thrombosis.